I encourage you to always be truthful when teaching and talking to your children. Explain the true story concerning the birth of Jesus. Talk about God's plan for salvation through Jesus. Talk about how Christmas is a day to celebrate God's gift to us. Regarding Santa Claus depending on the age of the children, explain to them how the true story of Santa Claus got started. I would encourage you to let them know that anything else is just like a fairy tale. There is no north pole with elves making gifts and flying reindeer. As Christians it is important to teach our children the truth concerning the Bible and how the world who doesn't believe in Jesus Christ often make up stories that take away from God's truth. Perhaps you could take what the world portrays as attributes of Santa and tell them the truth of the person and ministry of Jesus Christ and the Spirit of God at work in the world today.